Generally, a thermal flow detecting device detects flow-rate of fluid such as air in accordance with heat radiated from a heating element such as a heating resistance. The heating resistance includes a cylindrical bobbin and a pair of lead wires. The lead wires are inserted into both ends of the bobbin. The bobbin has the outer circumferential periphery around which a resistant wire is wound. The resistant wire is connected with the lead wires. The resistant wire and the lead wires are covered with a protective coat. For example, the protective coat is formed by painting or dipping glass slurry, drying the glass slurry, and subsequently, sintering the dried glass slurry. The protective coat includes an end surface coating covering an axially end surface of the bobbin. The end surface coating of the protective coat is in a substantially conical shape, and has large variation in thickness thereof such that the radially inner end thereof becomes thick and the radially outer end thereof becomes thin. Accordingly, variation in heat radiated from the heating resistance is large, and consequently, detection accuracy of the flow detecting device is low.
According to JP-A-8-110256, the axially end surface of the bobbin is in a tapered conical shape, so that variation in thickness of the protective coat is reduced around the axially end surface of the bobbin. In this structure, variation in heat radiated from the end surface of the bobbin can be reduced. Generally, the bobbin is formed of a heat resistive material such as ceramic since the bobbin needs to be resistive against high temperature generated from the heating resistance. It is costly to accurately form ceramic to be a conical shape defining the axial end surface of the bobbin.